1. Field of the Invention
The present invention relates to a method for preparing p-isopropenyl phenol of high purity and, more particularly, to a method for the preparation of p-isopropenyl phenol by dissolving or collecting in an organic solvent, p-isopropenyl phenol which is obtained either by subjecting dihydroxydiphenylpropane to thermal decomposition in the presence of a basic catalyst, or by thermally treating lower polymers or oligomers of p-isopropenyl phenol in the presence or absence of a basic catalyst. The dihydroxydiphenylpropane herein described is intended to mean 2,2-(4,4'-dihydroxydiphenyl)-propane or a mixture thereof with 2-(2-hydroxyphenyl)-2-(4'-hydroxyphenyl)-propane, and is hereinafter referred to simply as bisphenol A for the sake of brevity.
2. Description of the Prior Art
It is well known in the art that thermal treatment of bisphenol A results in the formation of phenol and p-isopropenyl phenol by a decomposition reaction. Moreover, it is also known that p-isopropenyl phenol can be obtained as a by-product when diisopropylbenzene is subjected to air-oxidation and then to acid decomposition to produce a dihydric phenol. However, it is difficult to isolate highly pure p-isopropenyl phenol monomer at a high yield since the monomer polymerizes readily, especially in the molten state.
P-Isopropenyl phenol has been hithertofore isolated by the process disclosed in British Patent Specification No. 905,994 wherein a decomposition reaction mixture of bisphenol A or the mixture from which phenol has been separated is thermally treated to form p-isopropenyl phenol gas, followed by rapid cooling of the resultant p-isopropenyl phenol gas to below the melting point thereof. However, the p-isopropenyl phenol thus obtained generally contains a large amount of p-isopropenyl phenol polymers. Accordingly, it is necessary to purify by recrystallization with a solvent such as cyclohexane or n-hexane for isolation of the p-isopropenyl phenol monomer, with the result that the yield of p-isopropenyl calculated on the basis of the starting bisphenol A employed is very low. As hereinbefore described, the p-isopropenyl phenol monomer tends to polymerize readily, especially in the molten state and, even in the solid state, non-crystallized or amorphous p-isopropenyl phenol slowly polymerizes in the vicinity of room temperature, and thus it is considered to be an unstable compound.
Under these circumstances, the present inventors undertook an intensive study to provide a method for preparing p-isopropenyl phenol of high purity and at a high yield. The compound is undesirably polymerizable not only at high temperatures but also in the vicinity of room temperature. It was found that although neither can the polymerization of p-isopropenyl phenol monomer be stopped by addition of a known radical polymerization inhibitor such as an alkylphenol including 2,6-di-tert-butyl-4-methylphenol, nor stabilization of the phenol be achieved by use of a non-polar solvent such as an aliphatic hydrocarbon or the like, an organic acid, or an amine or the like, the phenol is unexpectedly maintained stable even at temperatures higher than 100.degree. C. when dissolved in a polar solvent including an alcohol, an ester, or an acid amide. The present invention is based upon the above finding. p-Isopropenyl phenol which is dissolved in a polar solvent can be isolated from the solution by cooling or by adding thereto a poor solvent for the phenol, e.g., water. Moreover, the solution of p-isopropenyl phenol may be used as the starting material for subsequent reaction, i.e., the solution may be reacted with hydrogen peroxide in the presence of a strong acid to produce hydroquinone.